1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to an improved in-phase/quadrature-phase (I/Q) signal generation device for a wireless communication system.
2. Description of the Related Art
Due to the increasing of wireless communication equipment together with the wide spread user of wireless communications, attempts have been made to integrate the constituent elements of a transceiving device have a on one chip for miniaturization, low power consumption and low price. The constituent elements require reference signals of in-phase (I) and quadrature-phase (Q).
In a communication system using an orthogonal channel, I and Q channels are orthogonal to each other, but orthogonality therebetween is lost due to a defect caused when an element such as an oscillator is implemented. Therefore, a gain and a phase are unbalanced and a direct current (DC) error is caused, thereby deteriorating performance of the communication system.
A direct conversion receiver (DCR) technique is essentially required in a current environment where a variety of wireless communication standards coexist. In a conventional heterodyne receiver, a signal is amplified at 50 dB to 60 dB before its I/Q separation, and therefore, lower-gain amplification is required. Accordingly, a mismatching problem is considerable. Unlike this, in a DCR receiver, an input signal is amplified at 10 dB to 20 dB before its I/Q separation, and therefore, higher-gain amplification is required. Accordingly, an I/Q mismatching problem should be fully considered when designing a receiver.
FIG. 1 is a diagram illustrating a conventional DCR-based I/Q signal generation circuit. As shown in FIG. 1, an input signal is band-pass-filtered by a band-pass filter 101 and then amplified by a low noise amplifier 102. The amplified signal is mixed in mixers 104 and 106, which are separately installed on two signal lines with a local oscillation signal from a local oscillator 103. The mixed signals are output as I/Q signals through low-pass filters 107 and 108 and power amplifiers 109 and 110. The local oscillation signal is output to the mixer 106 for the Q signal, and is shifted 90° by a phase shifter 105 before being output to the mixer 104 for the I signal.
FIGS. 2A and 2B are graphs illustrating gain mismatching between the local oscillator and the phase shifter, and FIGS. 2C and 2D are graphs illustrating phase mismatching between the local oscillator and the phase shifter. It can be appreciated that the original signal points are distorted toward an I-axis and a Q-axis in a signal constellation.
In order to solve the mismatching problem, a frequency divider is generally used in a conventional oscillator. The frequency divider is being widely used even in a DCR system requiring an oscillation frequency different from a reference frequency. However, in an I/Q matching scheme using the frequency divider, if an input signal includes second harmonics, its I/Q signals suffer considerable phase mismatching, causing high power consumption and a difficulty in generating and separating high frequencies.
As another I/Q matching method, there is a known method using a resistor-capacitor (RC) polyphase filter. This can be used only for narrow-range control due to its high I/Q mismatching, and should be used together with a limiter to reduce the gain mismatching. This method is disadvantageous in that the RC polyphase filter reduces I/Q signal power.